I prefer 90/90-18 f and 100/90-18 rear. At the moment, I've got Avon Roadriders on my track bike, and they seem to stick OK. AFIK, the smallest Bridgestone BT45 rear is 110/90-18. A Pirelli / Bridgestone vendor is going to sponsor our rider school this year, and wants all us instructors to contract for his brands. So, I may run run a pair of BT45s in 90/90 and 110/90.